Separator within an EnumProperty
From the documentation for 2.9:

When an item only contains 4 items they define (identifier, name, description, number).
Separators may be added using None instead of a tuple. For dynamic values a callback can be passed which returns a list in the same format as the static list.

This is conspicuously absent in the documentation for 2.79, so there may not have been an easy way to do it in Python when this question was asked. But for newer versions, you can just use a straight None instead of a tuple.